Acid solutions are often employed to stimulate the production of fluids from sandstone and other siliceous formations surrounding oil and gas production wells by improving permeability.
The methods are generally referred to as acidizing processes and include the injection of an aqueous solution containing a mixture of hydrofluoric and hydrochloric acids into the formation and the subsequent production of the spent acid from the formation into the well bore.
Siliceous formations have been acidized by contact with mud acid. As used herein, “mud acid” refers to an aqueous solution of hydrofluoric acid (HF) and at least one acid selected from hydrochloric acid (HCl,) acetic acid (HAc) or formic acid; usually, the acid in addition to HF is HCl. As is well understood in the art, the derivation of the HCl and HF is not critical, so that “mud acid” may also include aqueous solutions of chemicals which react quickly to form HCl and HF, i.e., so that by the time the solution reaches the formation, the active ingredients are HF and HCl. The respective concentrations of HCl and HF may vary over wide ranges, with the lower limits being more a matter of practicality rather than operability, and the upper limits being a matter of mutual solubility of the two acids.
It has been reported that heated and pressurized acid is able to enter microfractures in the cemented annulus between the casing and formation resulting in significant acid damage to even latex-containing cement.
Laboratory test work has been reported indicating that acetic acid alone, or in combination with HF, dissolves much less cement than HCl and HCl—HF mixtures, while providing the same carbonate-dissolving ability. It is reported that the reaction of acetic acid with cement forms a protective skin that inhibits further acid attack.
The literature also reports that when laboratory samples of hardened slurry cement were subjected to attack by solutions containing HF, a surface layer of fluorite was observed to have formed. The fluorite was reported to be more soluble in HCl than in HAc.
Other laboratory tests undertaken on cement cubes reported that exposure to HCl—HF mixtures appeared to result in the formation of a protective skin of amorphous silica that slowed or inhibited the reaction with HCl. It was also reported that HF reacted with an HF-soluble portion of the cement, i.e., the amorphous silica component, so long as there was shear at the acid/cement interface to remove the protective skin formed on the cement.
It is also known from U.S. Pat. No. 3,543,856 that a formation to be treated with an aqueous solution of HCl and HF can be preflushed with an aqueous ammonium chloride solution to displace water containing metallic ions and to serve as a stabilizing agent for water sensitive clay.
As a result of these findings, acetic acid has been substituted for some or all of the HCl used in acid muds and other acidization compositions. The reduction in the adverse effects of acid treatments which include acetic acid is reported to be the formation of a deposit or protective layer that slows or minimizes the rate of attack of the other acids used in the well or formation-treating process.
It is therefore an object of the present invention to provide a process for pretreating an existing cement plug or other cement construction in a well bore in order to protect the cement from attack and deterioration when it is contacted during acidizing operations.
Another object of the invention is to provide a process that can quickly and effectively protect the surfaces of cement compositions in a well bore from the destructive effects of subsequent acid contact.
It is still another object of the invention to provide a process that can be utilized to preserve the integrity of cement seals and plugs under a variety of pre-existing conditions in the region of a well bore and formation that is to be subjected to acidization.
A further object of the invention is to provide a method and composition for forming protective precipitates on exposed cement surfaces that will eliminate or minimize deleterious effects of subsequent acidization treatments of the adjacent formation.